The Collaborative International Dictionary of English v.0.48:

Prepositional \Prep`o*si"tion*al\, a. [Cf. F.
   pr['e]positionnel.]
   Of or pertaining to a preposition; of the nature of a
   preposition. --Early. -- Prep`o*si"tion*al*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prepositionally
    adv 1: as a preposition